Adaptive weighting of reference pictures in video encoding

ABSTRACT

A video decoder, encoder, and corresponding methods for processing video signal data for an image block and a particular reference picture index to predict the image block are disclosed that utilize adaptive weighting of reference pictures to enhance video compression, where a decoder includes a reference picture weighting factor unit for determining a weighting factor corresponding to the particular reference picture index; an encoder includes a reference picture weighting factor assignor for assigning a weighting factor corresponding to the particular reference picture index; and a method for decoding includes receiving a reference picture index with the data that corresponds to the image block, determining a weighting factor for each received reference picture index, retrieving a reference picture for each index, motion compensating the retrieved reference picture, and multiplying the motion compensated reference picture by the corresponding weighting factor to form a weighted motion compensated reference picture.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional PatentApplication Serial No. 60/395,843 (Atty. Docket No. PU020340), entitled“Adaptive Weighting Of Reference Pictures In Video CODEC” and filed Jul.15, 2002, which is incorporated by reference herein in its entirety. Inaddition, this application claims the benefit of U.S. Provisional PatentApplication Serial No. 60/395,874 (Atty. Docket No. PU020339), entitled“Motion Estimation With Weighting Prediction” also filed Jul. 15, 2002,which is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

[0002] The present invention is directed towards video encoders and inparticular, towards adaptive weighting of reference pictures in videoencoders.

BACKGROUND OF THE INVENTION

[0003] Video data is generally processed and transferred in the form ofbit streams. Typical video compression coders and decoders (“CODECs”)gain much of their compression efficiency by forming a reference pictureprediction of a picture to be encoded, and encoding the differencebetween the current picture and the prediction. The more closely thatthe prediction is correlated with the current picture, the fewer bitsthat are needed to compress that picture, thereby increasing theefficiency of the process. Thus, it is desirable for the best possiblereference picture prediction to be formed.

[0004] In many video compression standards, including Moving PictureExperts Group (“MPEG”)-1, MPEG-2 and MPEG-4, a motion compensatedversion of a previous reference picture is used as a prediction for thecurrent picture, and only the difference between the current picture andthe prediction is coded. When a single picture prediction (“P” picture)is used, the reference picture is not scaled when the motion compensatedprediction is formed. When bidirectional picture predictions (“B”pictures) are used, intermediate predictions are formed from twodifferent pictures, and then the two intermediate predictions areaveraged together, using equal weighting factors of (½, ½) for each, toform a single averaged prediction. In these MPEG standards, the tworeference pictures are always one each from the forward direction andthe backward direction for B pictures.

SUMMARY OF THE INVENTION

[0005] These and other drawbacks and disadvantages of the prior art areaddressed by a system and method for adaptive weighting of referencepictures in video coders and decoders.

[0006] A video encoder, and corresponding methods for processing videosignal data for an image block and a particular reference picture indexto predict the image block are disclosed that utilize adaptive weightingof reference pictures to enhance video compression. An encoder includesa reference picture weighting factor assignor for assigning a weightingfactor to the particular reference picture index.

[0007] A corresponding method for encoding video video signal data foran image block includes receiving a substantially uncompressed imageblock and assigning a weighting factor for the image block correspondingto a particular reference picture having a corresponding index. Motionvectors are computed corresponding to the difference between the imageblock and the particular reference picture. A particular referencepicture is motion compensated in correspondence with the motion vectorsand the motion compensated reference picture is modified by the assignedweighting factor to form a weighted motion compensated referencepicture. The substantially uncompressed image block is compared to theweighted motion compensated reference picture, and a signal indicativeof the difference between the substantially uncompressed image block andthe weighted motion compensated reference picture along with thecorresponding index of the particular reference picture is encoded.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Adaptive weighting of reference pictures in video coders anddecoders in accordance with the principles of the present invention areshown in the following exemplary figures, in which:

[0009]FIG. 1 shows a block diagram for a standard video decoder;

[0010]FIG. 2 shows a block diagram for a video decoder with adaptivebi-prediction;

[0011]FIG. 3 shows a block diagram for a video decoder with referencepicture weighting in accordance with the principles of the presentinvention;

[0012]FIG. 4 shows a block diagram for a standard video encoder;

[0013]FIG. 5 shows a block diagram for a video encoder with referencepicture weighting in accordance with the principles of the presentinvention;

[0014]FIG. 6 shows a flowchart for a decoding process in accordance withthe principles of the present invention; and

[0015]FIG. 7 shows a flowchart for an encoding process in accordancewith the principles of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0016] The present invention presents an apparatus and method for motionvector estimation and adaptive reference picture weighting factorassignment. In some video sequences, in particular those with fading,the current picture or image block to be coded is more stronglycorrelated to a reference picture scaled by a weighting factor than tothe reference picture itself. Video CODECs without weighting factorsapplied to reference pictures encode fading sequences veryinefficiently. When weighting factors are used in encoding, a videoencoder needs to determine both weighting factors and motion vectors,but the best choice for each of these depends on the other, with motionestimation typically being the most computationally intensive part of adigital video compression encoder.

[0017] In the proposed Joint Video Team (“JVT”) video compressionstandard, each P picture can use multiple reference pictures to form apicture's prediction, but each individual motion block or 8×8 region ofa macroblock uses only a single reference picture for prediction. Inaddition to coding and transmitting the motion vectors, a referencepicture index is transmitted for each motion block or 8×8 region,indicating which reference picture is used. A limited set of possiblereference pictures is stored at both the encoder and decoder, and thenumber of allowable reference pictures is transmitted.

[0018] In the JVT standard, for bi-predictive pictures (also called “B”pictures), two predictors are formed for each motion block or 8×8region, each of which can be from a separate reference picture, and thetwo predictors are averaged together to form a single averagedpredictor. For bi-predictively coded motion blocks, the referencepictures can both be from the forward direction, both be from thebackward direction, or one each from the forward and backwarddirections. Two lists are maintained of the available reference picturesthat may used for prediction. The two reference pictures are referred toas the list 0 and list 1 predictors. An index for each reference pictureis coded and transmitted, re_id_l0 and ref_idx_l1, for the list 0 andlist 1 reference pictures, respectively. Joint Video Team (“JVT”)bi-predictive or “B” pictures allows adaptive weighting between the twopredictions, i.e.,

Pred=[(P0)(Pred0)]+[(P1)(Pred1)]+D,

[0019] where P0 and P1 are weighting factors, Pred0 and Pred1 are thereference picture predictions for list 0 and list 1 respectively, and Dis an offset.

[0020] Two methods have been proposed for indication of weightingfactors. In the first, the weighting factors are determined by thedirections that are used for the reference pictures. In this method, ifthe ref_idx_10 index is less than or equal to ref_idx_l1, weightingfactors of (½, ½) are used, otherwise (2, −1) factors are used.

[0021] In the second method offered, any number of weighting factors istransmitted for each slice. Then a weighting factor index is transmittedfor each motion block or 8×8 region of a macroblock that usesbidirectional prediction. The decoder uses the received weighting factorindex to choose the appropriate weighting factor, from the transmittedset, to use when decoding the motion block or 8×8 region. For example,if three weighting factors were sent at the slice layer, they wouldcorrespond to weight factor indices 0, 1 and 2, respectively.

[0022] The following description merely illustrates the principles ofthe invention. It will thus be appreciated that those skilled in the artwill be able to devise various arrangements that, although notexplicitly described or shown herein, embody the principles of theinvention and are included within its spirit and scope. Furthermore, allexamples and conditional language recited herein are principallyintended expressly to be only for pedagogical purposes to aid the readerin understanding the principles of the invention and the conceptscontributed by the inventor to furthering the art, and are to beconstrued as being without limitation to such specifically recitedexamples and conditions. Moreover, all statements herein recitingprinciples, aspects, and embodiments of the invention, as well asspecific examples thereof, are intended to encompass both structural andfunctional equivalents thereof. Additionally, it is intended that suchequivalents include both currently known equivalents as well asequivalents developed in the future, i.e., any elements developed thatperform the same function, regardless of structure.

[0023] Thus, for example, it will be appreciated by those skilled in theart that the block diagrams herein represent conceptual views ofillustrative circuitry embodying the principles of the invention.Similarly, it will be appreciated that any flow charts, flow diagrams,state transition diagrams, pseudocode, and the like represent variousprocesses which may be substantially represented in computer readablemedia and so executed by a computer or processor, whether or not suchcomputer or processor is explicitly shown.

[0024] The functions of the various elements shown in the figures may beprovided through the use of dedicated hardware as well as hardwarecapable of executing software in association with appropriate software.When provided by a processor, the functions may be provided by a singlededicated processor, by a single shared processor, or by a plurality ofindividual processors, some of which may be shared. Moreover, explicituse of the term “processor” or “controller” should not be construed torefer exclusively to hardware capable of executing software, and mayimplicitly include, without limitation, digital signal processor (“DSP”)hardware, read-only memory (“ROM”) for storing software, random accessmemory (“RAM”), and non-volatile storage. Other hardware, conventionaland/or custom, may also be included. Similarly, any switches shown inthe figures are conceptual only. Their function may be carried outthrough the operation of program logic, through dedicated logic, throughthe interaction of program control and dedicated logic, or evenmanually, the particular technique being selectable by the implementeras more specifically understood from the context.

[0025] In the claims hereof any element expressed as a means forperforming a specified function is intended to encompass any way ofperforming that function including, for example, a) a combination ofcircuit elements that performs that function or b) software in any form,including, therefore, firmware, microcode or the like, combined withappropriate circuitry for executing that software to perform thefunction. The invention as defined by such claims resides in the factthat the functionalities provided by the various recited means arecombined and brought together in the manner which the claims call for.Applicant thus regards any means that can provide those functionalitiesas equivalent to those shown herein.

[0026] As shown in FIG. 1, a standard video decoder is indicatedgenerally by the reference numeral 100. The video decoder 100 includes avariable length decoder (“VLD”) 110 connected in signal communicationwith an inverse quantizer 120. The inverse quantizer 120 is connected insignal communication with an inverse transformer 130. The inversetransformer 130 is connected in signal communication with a first inputterminal of an adder or summing junction 140, where the output of thesumming junction 140 provides the output of the video decoder 100. Theoutput of the summing junction 140 is connected in signal communicationwith a reference picture store 150. The reference picture store 150 isconnected in signal communication with a motion compensator 160, whichis connected in signal communication with a second input terminal of thesumming junction 140.

[0027] Turning to FIG. 2, a video decoder with adaptive bi-prediction isindicated generally by the reference numeral 200. The video decoder 200includes a VLD 210 connected in signal communication with an inversequantizer 220. The inverse quantizer 220 is connected in signalcommunication with an inverse transformer 230. The inverse transformer230 is connected in signal communication with a first input terminal ofa summing junction 240, where the output of the summing junction 240provides the output of the video decoder 200. The output of the summingjunction 240 is connected in signal communication with a referencepicture store 250. The reference picture store 250 is connected insignal communication with a motion compensator 260, which is connectedin signal communication with a first input of a multiplier 270.

[0028] The VLD 210 is further connected in signal communication with areference picture weighting factor lookup 280 for providing an adaptivebi-prediction (“ABP”) coefficient index to the lookup 280. A firstoutput of the lookup 280 is for providing a weighting factor, and isconnected in signal communication to a second input of the multiplier270. The output of the multiplier 270 is connected in signalcommunication to a first input of a summing junction 290. A secondoutput of the lookup 280 is for providing an offset, and is connected insignal communication to a second input of the summing junction 290. Theoutput of the summing junction 290 is connected in signal communicationwith a second input terminal of the summing junction 240.

[0029] Turning now to FIG. 3, a video decoder with reference pictureweighting is indicated generally by the reference numeral 300. The videodecoder 300 includes a VLD 310 connected in signal communication with aninverse quantizer 320. The inverse quantizer 320 is connected in signalcommunication with an inverse transformer 330. The inverse transformer330 is connected in signal communication with a first input terminal ofa summing junction 340, where the output of the summing junction 340provides the output of the video decoder 300. The output of the summingjunction 340 is connected in signal communication with a referencepicture store 350. The reference picture store 350 is connected insignal communication with a motion compensator 360, which is connectedin signal communication with a first input of a multiplier 370.

[0030] The VLD 310 is further connected in signal communication with areference picture weighting factor lookup 380 for providing a referencepicture index to the lookup 380. A first output of the lookup 380 is forproviding a weighting factor, and is connected in signal communicationto a second input of the multiplier 370. The output of the multiplier370 is connected in signal communication to a first input of a summingjunction 390. A second output of the lookup 380 is for providing anoffset, and is connected in signal communication to a second input ofthe summing junction 390. The output of the summing junction 390 isconnected in signal communication with a second input terminal of thesumming junction 340.

[0031] As shown in FIG. 4, a standard video encoder is indicatedgenerally by the reference numeral 400. An input to the encoder 400 isconnected in signal communication with a non-inverting input of asumming junction 410. The output of the summing junction 410 isconnected in signal communication with a block transformer 420. Thetransformer 420 is connected in signal communication with a quantizer430. The output of the quantizer 430 is connected in signalcommunication with a variable length coder (“VLC”) 440, where the outputof the VLC 440 is an externally available output of the encoder 400.

[0032] The output of the quantizer 430 is further connected in signalcommunication with an inverse quantizer 450. The inverse quantizer 450is connected in signal communication with an inverse block transformer460, which, in turn, is connected in signal communication with areference picture store 470. A first output of the reference picturestore 470 is connected in signal communication with a first input of amotion estimator 480. The input to the encoder 400 is further connectedin signal communication with a second input of the motion estimator 480.The output of the motion estimator 480 is connected in signalcommunication with a first input of a motion compensator 490. A secondoutput of the reference picture store 470 is connected in signalcommunication with a second input of the motion compensator 490. Theoutput of the motion compensator 490 is connected in signalcommunication with an inverting input of the summing junction 410.

[0033] Turning to FIG. 5, a video encoder with reference pictureweighting is indicated generally by the reference numeral 500. An inputto the encoder 500 is connected in signal communication with anon-inverting input of a summing junction 510. The output of the summingjunction 510 is connected in signal communication with a blocktransformer 520. The transformer 520 is connected in signalcommunication with a quantizer 530. The output of the quantizer 530 isconnected in signal communication with a VLC 540, where the output ofthe VLC 440 is an externally available output of the encoder 500.

[0034] The output of the quantizer 530 is further connected in signalcommunication with an inverse quantizer 550. The inverse quantizer 550is connected in signal communication with an inverse block transformer560, which, in turn, is connected in signal communication with areference picture store 570. A first output of the reference picturestore 570 is connected in signal communication with a first input of areference picture weighting factor assignor 572. The input to theencoder 500 is further connected in signal communication with a secondinput of the reference picture weighting factor assignor 572. The outputof the reference picture weighting factor assignor 572, which isindicative of a weighting factor, is connected in signal communicationwith a first input of a motion estimator 580. A second output of thereference picture store 570 is connected in signal communication with asecond input of the motion estimator 580.

[0035] The input to the encoder 500 is further connected in signalcommunication with a third input of the motion estimator 580. The outputof the motion estimator 580, which is indicative of motion vectors, isconnected in signal communication with a first input of a motioncompensator 590. A third output of the reference picture store 570 isconnected in signal communication with a second input of the motioncompensator 590. The output of the motion compensator 590, which isindicative of a motion compensated reference picture, is connected insignal communication with a first input of a multiplier 592. The outputof the reference picture weighting factor assignor 572, which isindicative of a weighting factor, is connected in signal communicationwith a second input of the multiplier 592. The output of the multiplier592 is connected in signal communication with an inverting input of thesumming junction 510.

[0036] Turning now to FIG. 6, an exemplary process for decoding videosignal data for an image block is indicated generally by the referencenumeral 600. The process includes a start block 610 that passes controlto an input block 612. The input block 612 receives the image blockcompressed data, and passes control to an input block 614. The inputblock 614 receives at least one reference picture index with the datafor the image block, each reference picture index corresponding to aparticular reference picture. The input block 614 passes control to afunction block 616, which determines a weighting factor corresponding toeach of the received reference picture indices, and passes control to anoptional function block 617. The optional function block 617 determinesan offset corresponding to each of the received reference pictureindices, and passes control to a function block 618. The function block618 retrieves a reference picture corresponding to each of the receivedreference picture indices, and passes control to a function block 620.The function block 620, in turn, motion compensates the retrievedreference picture, and passes control to a function block 622. Thefunction block 622 multiplies the motion compensated reference pictureby the corresponding weighting factor, and passes control to an optionalfunction block 623. The optional function block 623 adds the motioncompensated reference picture to the corresponding offset, and passescontrol to a function block 624. The function block 624, in turn, formsa weighted motion compensated reference picture, and passes control toan end block 626.

[0037] Turning now to FIG. 7, an exemplary process for encoding videosignal data for an image block is indicated generally by the referencenumeral 700. The process includes a start block 710 that passes controlto an input block 712. The input block 712 receives substantiallyuncompressed image block data, and passes control to a function block714. The function block 714 assigns a weighting factor for the imageblock corresponding to a particular reference picture having acorresponding index. The function block 714 passes control to anoptional function block 715. The optional function block 715 assigns anoffset for the image block corresponding to a particular referencepicture having a corresponding index. The optional function block 715passes control to a function block 716, which computes motion vectorscorresponding to the difference between the image block and theparticular reference picture, and passes control to a function block718. The function block 718 motion compensates the particular referencepicture in correspondence with the motion vectors, and passes control toa function block 720. The function block 720, in turn, multiplies themotion compensated reference picture by the assigned weighting factor toform a weighted motion compensated reference picture, and passes controlto an optional function block 721. The optional function block 721, inturn, adds the motion compensated reference picture to the assignedoffset to form a weighted motion compensated reference picture, andpasses control to a function block 722. The function block 722 subtractsthe weighted motion compensated reference picture from the substantiallyuncompressed image block, and passes control to a function block 724.The function block 724, in turn, encodes a signal with the differencebetween the substantially uncompressed image block and the weightedmotion compensated reference picture along with the corresponding indexof the particular reference picture, and passes control to an end block726.

[0038] In the present exemplary embodiment, for each coded picture orslice, a weighting factor is associated with each allowable referencepicture that blocks of the current picture can be encoded with respectto. When each individual block in the current picture is encoded ordecoded, the weighting factor(s) and offset(s) that correspond to itsreference picture indices are applied to the reference prediction toform a weight predictor. All blocks in the slice that are coded withrespect to the same reference picture apply the same weighting factor tothe reference picture prediction.

[0039] Whether or not to use adaptive weighting when coding a picturecan be indicated in the picture parameter set or sequence parameter set,or in the slice or picture header. For each slice or picture that usesadaptive weighting, a weighting factor may be transmitted for each ofthe allowable reference pictures that may be used for encoding thisslice or picture. The number of allowable reference pictures istransmitted in the slice header. For example, if three referencepictures can be used to encode the current slice, up to three weightingfactors are transmitted, and they are associated with the referencepicture with the same index.

[0040] If no weighting factors are transmitted, default weights areused. In one embodiment of the current invention, default weights of (½,½) are used when no weighting factors are transmitted. The weightingfactors may be transmitted using either fixed or variable length codes.

[0041] Unlike typical systems, each weighting factor that is transmittedwith each slice, block or picture corresponds to a particular referencepicture index. Previously, any set of weighting factors transmitted witheach slice or picture were not associated with any particular referencepictures. Instead, an adaptive bi-prediction weighting index wastransmitted for each motion block or 8×8 region to select which of theweighting factors from the transmitted set was to be applied for thatparticular motion block or 8×8 region.

[0042] In the present embodiment, the weighting factor index for eachmotion block or 8×8 region is not explicitly transmitted. Instead, theweighting factor that is associated with the transmitted referencepicture index is used. This dramatically reduces the amount of overheadin the transmitted bitstream to allow adaptive weighting of referencepictures.

[0043] This system and technique may be applied to either Predictive “P”pictures, which are encoded with a single predictor, or to Bi-predictive“B” pictures, which are encoded with two predictors. The decodingprocesses, which are present in both encoder and decoders, are describedbelow for the P and B picture cases. Alternatively, this technique mayalso be applied to coding systems using the concepts similar to I, B,and P pictures.

[0044] The same weighting factors can be used for single directionalprediction in B pictures and for bi-directional prediction in Bpictures. When a single predictor is used for a macroblock, in Ppictures or for single directional prediction in B pictures, a singlereference picture index is transmitted for the block. After the decodingprocess step of motion compensation produces a predictor, the weightingfactor is applied to predictor. The weighted predictor is then added tothe coded residual, and clipping is performed on the sum, to form thedecoded picture. For use for blocks in P pictures or for blocks in Bpictures that use only list 0 prediction, the weighted predictor isformed as:

Pred=W0*Pred0+D0   (1)

[0045] where W0 is the weighting factor associated with the list 0reference picture, D0 is the offset associated with the list 0 referencepicture, and Pred0 is the motion-compensated prediction block from thelist 0 reference picture.

[0046] For use for blocks in B pictures which use only list 0prediction, the weighted predictor is formed as:

Pred=W1*Pred1++D1   (2)

[0047] where W1 is the weighting factor associated with the list 1reference picture, D0 is the offset associated with the list 1 referencepicture, and Pred1 is the motion-compensated prediction block from thelist 1 reference picture.

[0048] The weighted predictors may be clipped to guarantee that theresulting values will be within the allowable range of pixel values,typically 0 to 255. The precision of the multiplication in the weightingformulas may be limited to any pre-determined number of bits ofresolution.

[0049] In the bi-predictive case, reference picture indexes aretransmitted for each of the two predictors. Motion compensation isperformed to form the two predictors. Each predictor uses the weightingfactor associated with its reference picture index to form two weightedpredictors. The two weighted predictors are then averaged together toform an averaged predictor, which is then added to the coded residual.

[0050] For use for blocks in B pictures that use list 0 and list 1predictions, the weighted predictor is formed as:

Pred=(P0*Pred0+D0+P1*Pred1+D1)/2   (3)

[0051] Clipping may be applied to the weighted predictor or any of theintermediate values in the calculation of the weighted predictor toguarantee that the resulting values will be within the allowable rangeof pixel values, typically 0 to 255.

[0052] Thus, a weighting factor is applied to the reference pictureprediction of a video compression encoder and decoder that uses multiplereference pictures. The weighting factor adapts for individual motionblocks within a picture, based on the reference picture index that isused for that motion block. Because the reference picture index isalready transmitted in the compressed video bitstream, the additionaloverhead to adapt the weighting factor on a motion block basis isdramatically reduced. All motion blocks that are coded with respect tothe same reference picture apply the same weighting factor to thereference picture prediction.

[0053] These and other features and advantages of the present inventionmay be readily ascertained by one of ordinary skill in the pertinent artbased on the teachings herein. It is to be understood that the teachingsof the present invention may be implemented in various forms ofhardware, software, firmware, special purpose processors, orcombinations thereof.

[0054] Most preferably, the teachings of the present invention areimplemented as a combination of hardware and software. Moreover, thesoftware is preferably implemented as an application program tangiblyembodied on a program storage unit. The application program may beuploaded to, and executed by, a machine comprising any suitablearchitecture. Preferably, the machine is implemented on a computerplatform having hardware such as one or more central processing units(“CPU”), a random access memory (“RAM”), and input/output (“I/O”)interfaces. The computer platform may also include an operating systemand microinstruction code. The various processes and functions describedherein may be either part of the microinstruction code or part of theapplication program, or any combination thereof, which may be executedby a CPU. In addition, various other peripheral units may be connectedto the computer platform such as an additional data storage unit and aprinting unit.

[0055] It is to be further understood that, because some of theconstituent system components and methods depicted in the accompanyingdrawings are preferably implemented in software, the actual connectionsbetween the system components or the process function blocks may differdepending upon the manner in which the present invention is programmed.Given the teachings herein, one of ordinary skill in the pertinent artwill be able to contemplate these and similar implementations orconfigurations of the present invention.

[0056] Although the illustrative embodiments have been described hereinwith reference to the accompanying drawings, it is to be understood thatthe present invention is not limited to those precise embodiments, andthat various changes and modifications may be effected therein by one ofordinary skill in the pertinent art without departing from the scope orspirit of the present invention. All such changes and modifications areintended to be included within the scope of the present invention as setforth in the appended claims.

What is claimed is:
 1. A video encoder for encoding video signal datafor an image block and a particular reference picture index, the encodercomprising a reference picture weighting factor assignor for assigning aweighting factor corresponding to the particular reference pictureindex.
 2. A video encoder as defined in claim 1, further comprising areference picture store in signal communication with the referencepicture weighting factor assignor for providing a reference picturecorresponding to the particular reference picture index.
 3. A videoencoder as defined in claim 1, further comprising a variable lengthcoder in signal communication with the reference picture weightingfactor assignor for providing the particular reference picture weightingfactor to the variable length coder.
 4. A video encoder as defined inclaim 1, further comprising a motion compensation unit in signalcommunication with the reference picture weighting factor assignor forproviding motion compensated reference pictures responsive to thereference picture weighting factor assignor.
 5. A video encoder asdefined in claim 4, further comprising a multiplier in signalcommunication with the motion compensation unit and the referencepicture weighting factor assignor for applying a weighting factor to amotion compensated reference picture.
 6. A video encoder as defined inclaim 5 usable with bi-predictive picture predictors, the encoderfurther comprising prediction means for forming first and secondpredictors from two different reference pictures.
 7. A video encoder asdefined in claim 6 wherein the two different reference pictures are bothfrom the same direction relative to the image block.
 8. A method forencoding video signal data for an image block, the method comprising:receiving a substantially uncompressed image block; assigning aweighting factor for the image block corresponding to a particularreference picture having a corresponding index; computing motion vectorscorresponding to the difference between the image block and theparticular reference picture; motion compensating the particularreference picture in correspondence with the motion vectors; modifyingthe motion compensated reference picture by the assigned weightingfactor to form a weighted motion compensated reference picture;comparing the weighted motion compensated reference picture to thesubstantially uncompressed image block; and encoding a signal indicativeof the difference between the substantially uncompressed image block andthe weighted motion compensated reference picture along with thecorresponding index of the particular reference picture.
 9. A method asdefined in claim 8 wherein computing motion vectors comprises: testingwithin a search region for every displacement within a pre-determinedrange of offsets relative to the image block; calculating at least oneof the sum of the absolute difference and the mean squared error of eachpixel in the image block with a motion compensated reference picture;and selecting the offset with the lowest sum of the absolute differenceand mean squared error as the motion vector.
 10. A method as defined inclaim 8 wherein bi-predictive picture predictors are used, the methodfurther comprising: assigning a second weighting factor for the imageblock corresponding to a second particular reference picture having asecond corresponding index; computing motion vectors corresponding tothe difference between the image block and the second particularreference picture; motion compensating the second particular referencepicture in correspondence with the motion vectors; multiplying themotion compensated second reference picture by the assigned secondweighting factor to form a weighted motion compensated second referencepicture; subtracting the weighted motion compensated second referencepicture from the substantially uncompressed image block; and encoding asignal indicative of the difference between the substantiallyuncompressed image block and the weighted motion compensated secondreference picture along with the corresponding index of the secondparticular reference picture.
 11. A method as defined in claim 10wherein the two different reference pictures are both from the samedirection relative to the image block.
 12. A method as defined in claim10 wherein computing motion vectors comprises: testing within a searchregion for every displacement within a pre-determined range of offsetsrelative to the image block; calculating at least one of the sum of theabsolute difference and the mean squared error of each pixel in theimage block with a first motion compensated reference picturecorresponding to the first predictor; selecting an offset with thelowest sum of the absolute difference and mean squared error as themotion vector for the first predictor; calculating at least one of thesum of the absolute difference and the mean squared error of each pixelin the image block with a second motion compensated reference picturecorresponding to the second predictor; and selecting an offset with thelowest sum of the absolute difference and mean squared error as themotion vector for the second predictor.